The present invention relates generally to a reciprocating motor with a uni-directional fluid flow path. The present device may be employed to convert fluid energy into useful mechanical work for any machine, such as a reciprocating piston pump. The present device is particularly advantageous for applications such as cryogenic pumps where the continuous uni-directional flow of fluid reduces the effect of heat transfer between the fluid within the reciprocating motor and the cryogenic apparatus.
Conventional double-acting reciprocating motors use differential fluid pressure applied to a piston to cause reciprocating movement of the piston within a motor cylinder. Chambers on either side of the piston are equipped with respective fluid inlets and outlets that are controlled by external valves.
The piston moves to expand the volume of a first chamber by opening the inlet valve and closing the outlet valve associated with the first chamber while closing the inlet valve and opening the outlet valve associated with the second chamber on the opposite side of the piston. High-pressure fluid enters the first chamber through the open inlet valve while fluid is drained from the second chamber through the open outlet valve.
To move the piston in the opposite direction, the valve settings are reversed so that high-pressure fluid fills the second chamber and fluid is drained from the first chamber.
This type of reciprocating motor is known as a xe2x80x9cdouble-actingxe2x80x9d motor because fluid pressure is employed to move the piston in both directions and the piston rod extending from the reciprocating motor can perform mechanical work when traveling in both directions. A double-acting reciprocating motor is needed to drive a double-acting cryogenic pump that is designed to compress a cryogen with each piston stroke. That is, the pump piston compresses cryogen in both directions.
U.S. Pat. No. 4,458,579 (the ""579 patent) discloses a motor for actuating a downhole pump in an oil well. The motor employs fluid pressure to raise the piston. At the top of the piston stroke a valve opens to allow the fluid to flow through the piston. The ""579 patent discloses a motor with uni-directional fluid flow, but the motor is a single-acting motor that relies upon the force of gravity for downward movement of the piston. The motor has no valve at the fluid outlet for allowing fluid pressure to build in the cylinder space above the piston during the down-stroke.
U.S. Pat. No. 5,341,723 (the ""723 patent) discloses a reciprocating air motor with a uni-directional air flow through the motor cylinder. The ""723 patent discloses an internal venting arrangement whereby at the end of the piston stroke a groove in the cylinder wall allows the pressurized air to enter an internal chamber within the piston to open a valve to vent the pressurized air through the piston. However, like the ""579 patent, the ""723 patent does not disclose a double-acting reciprocating motor in that the pressurized air that passes through the piston is simply vented and a spring is employed to push the piston back to the starting position.
U.S. Pat. No. 5,203,251 (the ""251 patent) discloses an air motor that has an air inlet and outlet on the same side of the piston. The air exits the motor through a bore formed in the piston rod. This arrangement may be suitable for air motors where the air is typically vented after exiting the motor. However, removing the fluid through the piston rod results in a more complicated arrangement in a closed loop system, which is typically the case when the fluid is a hydraulic oil or other liquid. When a high pressure fluid is employed, for example, for applications such as driving cryogenic pumps, an essentially incompressible liquid is typically employed instead of a gaseous fluid, such as air. Discharging the air through the piston rod, as disclosed by the ""251 patent, also increases the time that the fluid is within the motor assembly and directs the fluid back to the same side as the inlet before the fluid is ultimately recovered in a closed-loop system. If this arrangement is employed for driving a cryogenic pump, the fluid would be directed back to the xe2x80x9ccoldxe2x80x9d side before exiting the motor.
For cryogenic applications, the fluid is typically a liquid such as a hydraulic oil, which is virtually incompressible and which also helps to lubricate the piston and cylinder. A particular problem with known double-acting reciprocating motors, which are employed to drive cryogenic pumps, is that there is a potential for the liquid within the motor cylinder nearest the cryogenic pump to become frozen. The problem is exacerbated if the same liquid is repeatedly returned to the xe2x80x9ccoldxe2x80x9d side of the reciprocating motor without being directed back to the fluid reservoir or to the xe2x80x9cwarmxe2x80x9d side of the motor that is further from the cryogenic pump. Thermal insulation is typically provided to shield the liquid from the cooling effect of the cryogenic pump. However, thermal insulation interposed between the cryogenic pump and the reciprocating motor adds to the weight, bulk and overall length of the pump and motor assembly. Furthermore, it is difficult to completely eliminate heat transfer because the piston rod assembly acts as a thermal conductor between the reciprocating motor and the cryogenic apparatus.
If actuation liquid is cooled so that it freezes within the reciprocating motor cylinder, severe damage may be caused to the motor and/or piston rod.
An objective of the present device is to provide a reciprocating motor with a uni-directional fluid flow path for applications that employ a double-acting motor. A uni-directional flow path allows fluid to progressively flow through the motor. When such a motor is employed for driving a cryogenic apparatus, this prevents the fluid from being exposed for prolonged periods to the xe2x80x9ccoldxe2x80x9d end of the motor, which is coupled to the cryogenic apparatus. This is advantageous for reducing the susceptibility of the fluid to freezing. The fluid flowing through the motor may also increase in temperature as a result of heat generated by the mechanical motor apparatus. Accordingly, because the uni-directional flow path generally results in the fluid flowing away from the cold end towards the opposite xe2x80x9cwarmxe2x80x9d end, this arrangement helps to reduce the transfer of heat from the motor apparatus to a cryogenic apparatus, which is maintained at cryogenic temperatures.
A double-acting reciprocating motor with a uni-directional flow path is provided that comprises:
a housing having a hollow cylinder disposed between a cylinder head and a cylinder base;
a piston disposed within the cylinder between the cylinder head and cylinder base, the piston having a first pressure surface area and a second pressure surface area opposite to and larger than the first pressure surface area;
a piston shaft operatively associated with the piston and extending from the piston through the cylinder base;
a fluid inlet for directing uni-directional fluid flow into a first chamber, the first chamber defined within the cylinder between the cylinder base and the first surface area;
a fluid outlet for draining fluid from a second chamber, the second chamber defined within the cylinder between the cylinder head and the second surface area;
a fluid passageway comprising a fluid passage disposed within the piston, the fluid passageway fluidly connecting the first chamber to the second chamber;
a pass-through valve associated with the cylinder head for selectively opening and closing the fluid passageway; and
an outlet valve that is openable for draining fluid from the second chamber when the pass-through valve is in the closed position.
The disclosed motor may employ a gaseous or liquid actuation fluid, but as already mentioned, when the motor is employed for applications that require a high pressure actuation fluid, it is preferable to use a liquid since it is substantially incompressible. For example, if the motor is employed to drive a double-acting cryogenic pump, such an application is especially suitable for the present motor driven by a high pressure liquid.
In the present embodiments of the motor, the pass-through valve and the outlet valve are preferably electronically controlled.
In some preferred embodiments, the fluid passage is defined by a well formed within the piston with an open end associated with the second pressure surface area and a fluid port through which fluid is flowable from the first chamber to the interior of the well. The fluid passageway further comprises:
a hollow member extending from the cylinder head and aligned with the well, whereby fluid is flowable from the well through the hollow member to the pass-through valve;
a seal between the hollow member and the well, sealing against fluid flow between the well and the second chamber; and
a conduit through which fluid is flowable from the pass-through valve to the second chamber.
In one embodiment, the fluid conduit connected to the outlet of the pass-through valve communicates with the fluid outlet of the second chamber upstream of the motor outlet valve. According to this embodiment, the fluid outlet of the second chamber also acts as the fluid inlet to the second chamber when the outlet valve is closed and the pass-through valve is open.
In another embodiment, the pass-through valve comprises a flow control mechanism disposed within the hollow member. Openings formed in the hollow member between the flow control mechanism and the cylinder head act as the fluid conduits for introducing the fluid into the second chamber.
In a further embodiment of the reciprocating motor, a fluid passage communicates between the first chamber and the interior of a hollow member that extends from the second pressure surface area of the piston and into a well formed in the cylinder head. The pass-through valve is positioned to receive fluid from the well, and the fluid passageway further comprises:
a seal between the hollow member and the well, sealing against fluid flow between the well and the second chamber; and
a conduit through which fluid is flowable from an outlet of the pass-through valve into the second chamber.
Instead of employing a rigid, fixed-length hollow member and a sleeve, the fluid passage leading from the first chamber may communicate with the interior of a hollow telescoping member that extends from the second pressure surface area of the piston to the cylinder head. The pass-through valve is positioned to receive fluid from the hollow telescoping member and the overall length of the motor axis can be reduced by eliminating the sleeve and providing only a well within the cylinder head to receive the collapsed hollow telescoping member. The fluid passageway further comprises a conduit through which fluid is flowable from an outlet of the pass-through valve into the second chamber.
A common feature of the above-described embodiments is the uni-directional fluid flow path. During operation, fluid is continuously introduced into the motor assembly through the inlet port associated with the cylinder base and the first chamber. Fluid is drained from the motor only from the opposite end, through the outlet port associated with the cylinder head and the second chamber. Another advantage of the present embodiments is that conventional valves may be employed for the pass-through valve and the outlet valve, which are both associated with the cylinder head, which is furthest from the cold end, and where they are accessible for maintenance and replacement without requiring disassembly of the cylinder assembly. The pass-through valve may be located outside of the cylinder assembly or within a segregated portion of the second chamber that may be made accessible through a removable cap in the cylinder head. That is, for the pass-through valve and the outlet valve, the valve mechanism and actuator are both located either outside the motor body, in the cylinder head, or in a segregated portion of the second chamber proximate to the cylinder head. Neither of the valves or their actuators are associated with the cold end of the motor.
Also provided is a method of operating a double-acting reciprocating motor with a uni-directional flow path, such as the motors described above. The motor comprises a movable piston disposed within a cylinder between a cylinder head and a cylinder base, defining a first variable volume chamber between the cylinder base and a first piston pressure surface and a second variable volume chamber between the cylinder head and a second piston pressure surface. The second piston pressure surface is larger than the first piston pressure surface and a pass-through valve is operable to allow fluid to flow from the first chamber to the second chamber. An outlet valve is operable to drain fluid from the second chamber. The method comprises:
introducing the actuation fluid through an inlet port into the first chamber to cause reciprocating motion of the piston;
closing the pass-through valve and opening the outlet valve when the piston approaches the cylinder base so that fluid pressure within the first chamber causes the piston to move towards the cylinder head while fluid is drained from the second chamber through the outlet valve;
opening the pass-through valve and closing the outlet valve when the piston approaches the cylinder head so that fluid pressure within the second chamber causes the piston to move towards the cylinder base; and
electronically controlling the respective opening and closing of the pass-through valve and the outlet valve.
An advantage of this method is that reliable electronic controllers and valve actuators may be employed to control the opening and closing of the valves, thereby reducing or eliminating the need for mechanical actuator assemblies disposed within the cylinder assembly, which may require customized components and more disassembly for service and replacement purposes.
Employing an embodiment of the present apparatus in another embodiment of the method, the method comprises:
introducing the fluid into the first chamber through an inlet port associated with the cylinder base to cause reciprocating motion of the piston;
closing a pass-through valve to prevent fluid flow from the first chamber to the second chamber and opening an outlet valve associated with the cylinder head to allow fluid pressure within the first chamber to act on the piston whereby the piston moves towards the cylinder head while fluid is drained from the second chamber through the open outlet valve; and
opening the pass-through valve and closing the outlet valve to allow fluid pressure within the second chamber to act on the piston whereby the piston moves towards the cylinder base while fluid flows from the first chamber to the second chamber through the open pass-through valve;
whereby the fluid flows progressively into the first chamber through the inlet port, then through the pass-through valve to the second chamber, and then out through the outlet valve.
A feature of the disclosed method is directing the fluid flow progressively through the motor apparatus, to simplify the flow path whereby fluid flowing through the motor does not reverse direction in any of the fluid passages, unlike conventional double-acting motors described above, which may reverse the direction of fluid flow and direct the same fluid repeatedly into the same chamber. The present method is particularly advantageous to reduce the heat transfer between the fluid and apparatus driven by the motor. For example, as noted previously, heat transfer is an important consideration when the motor is coupled to a cryogenic pump for driving a reciprocating pump piston.